During drilling operations, overbalanced drilling fluid pressure and filtrate leak-off can cause pressure build-up in the local formation around the wellbore. This leak-off and pressure build-up, known as supercharging, is generally accompanied by filter cake deposition and growth at the sand face of the wellbore, and also changes within the formation due to mud filtrate invasion. The filter cake hydraulic conductivity changes with time, affecting the pressure drop across it, and the pressure behind it at the sand face. This makes it difficult to estimate the amount of supercharging and hence the far field formation pressure, even if the history of drilling fluid circulation and local wellbore pressure variation is known. A summary of the phenomenology of drilling fluid filtrate leak-off, during and after drilling, may be found in E. J. Fordham, D. F. Allen & H. K. J. Ladva “The Principle of a Critical Invasion Rate and its Implications for Log Interpretation” Society of Petroleum Engineers (SPE) paper 22539, (1991).
Conventional formation pressure measurements, made with formation testing tools which probe the formation from the wellbore, often read high compared to the actual reservoir pressure far from the borehole, due to the supercharging effect. This is a particular problem in relatively low permeability reservoirs (below approximately 1 mD/cp). Significant difficulties are related to (1) poor knowledge of filter cake and formation physical properties, (2) the long timescales over which wellbores are typically exposed to overbalanced pressure, and (3) practical time constraints, which often require pressure measurements to be carried out during a rather short time compared to the time of pressure build-up around a wellbore. Even with known transient pressure testing techniques, these difficulties make it problematical to sense the far field formation pressure at the boundary of the pressure build-up zone because of the slow pressure wave propagation inherent in low permeability formations.
Various schemes have been put forward for making supercharging corrections to wireline pressure measurements: see, for example, SPE paper 36524 (“Supercharge Pressure Compensation Using a New Wireline Testing Method and Newly Developed Early Time Spherical Flow Model”), EP-A-0897049, and SPE paper 64227 (“Adverse Effects of Poor Mudcake Quality: A Supercharging and Fluid Sampling Study”). SPE papers 12962 (“The Effect of Filtrate Invasion and Formation Wettability on Repeat Formation Tester Measurements”) and 13287 (“The Analysis of the Invaded Zone Characteristics and Their Influence on Wireline Log and Well-Test Interpretation”) discuss the role of two-phase flow effects, which are largely ignored in the later supercharging interpretation schemes.
U.S. Pat. No. 5,233,866 describes a tool and method for supercharging correction, in which filtrate leak-off rate is determined from the rate at which pressure decays in a closed volume of drilling fluid in contact with the filtercake and via that the formation.
U.S. Pat. No. 5,644,076 proposes a method for obtaining filtercake properties and filtrate leak-off rates from the rate of decay of pressure in a closed chamber of mud communicating with the formation through the filtercake. This information is then used in the determination of the supercharging pressure. U.S. Pat. No. 5,602,334 describes tool operation and interpretation methodologies for obtaining permeability and sandface pressure in low permeability formations.
SPE papers 84088 (“Formation Pressure Testing During Drilling: Challenges and Benefits”) and 87091 (“Field Experience With a New Formation Pressure Testing-During-Drilling Tool”) discuss Baker Hughes' TesTrak™ pressure measurement-while-drilling tool. The 87091 paper remarks on observations of supercharging in the field, but does not present a correction methodology. SPE paper 87090 (“Formation Pressure Testing In the Dynamic Drilling Environment”) discusses Halliburton's GeoTap™ pressure measurement-while-drilling tool, discusses interpretation, and presents some simulations to show that significant supercharging is restricted to sub-milliDarcy formations. SPE paper 87092 (“An LWD Formation Pressure Test Tool (DFT) Refined the Otter Field Development Strategy”) discusses PathFinder Energy Services' pressure measurement-while-drilling technique. This differs in design from other offerings, in that a dual packer system, rather than a small probe, is used to connect the tool to the formation. Little is said about supercharging or interpretation.
SPE paper 50128 (“New Techniques in Wireline Formation Testing in Tight Reservoirs”) discusses the supercharging correction method of EP-A-0897049 (“Method and apparatus for determining formation pressure”), and comments that a) real measurements performed at a number of different wellbore pressures sometimes show evidence of cake compactability (because of variation of cake permeability with pressure), and b) “this would lead to significant errors in the calculation of the un-supercharged pressure”. The document does not discuss how this error might be eliminated.
While conventional tools and techniques can often work well in relatively high permeability formations, where supercharging easily dissipates, there is still a need for a technique for estimating the amount of supercharging that can be successfully employed in relatively low permeability formations.